1. Field of the Invention
The invention relates to a device for reversible optical data storage, employing polymeric liquid crystals.
2. Discussion of the Background:
In certain substances there exist intermediate phases between the solid crystalline phase and the fluid melt. The term "fluid melt" is hereinafter referred to as the "isotropic melt". These intermediate phases combine properties of the ordered crystalline state and the unordered molten state, with regard to structural and dynamic phenomena. Thus, these phases are fluid but they have, for example, optical properties which are characteristic of most crystalline or partially crystalline materials, i.e., they are birefringent. Quite understandably, they are described as "intermediate phases", "mesophases", or "liquid crystal phases". These intermediate phases can be obtained by varying the temperature (wherewith they are designated as "thermotropic liquid crystals") or in solution by varying the concentration. Hereinafter, only thermotropic liquid crystals will be considered. To characterize these intermediate phases, one generally specifies the transition temperatures (determined calorimetrically or by a polarized light microscope) of transitions from the crystalline state to the liquid crystal state, and from the liquid crystal state to the isotropic melt ("clear point"). In addition, if different liquid crystal states are present, all transition temperatures are given.
The occurrence of mesophases associated with peculiarities in the molecular geometry. Spherically symmetric molecules cannot form mesophases, but molecules which can be described as having a roughly cylindrical or disc shape can form mesophases. Such molecules may be rigid, and the ratio of their maximum to minimum dimension (e.g., cylinder length to cylinder diameter) must clearly exceed a critical ratio value of about 3.
In the simplest case for cylindrically shaped molecules, the structure of such mesophases is characterized in that the so-called "nematic" phase the molecular centers have an unordered distribution as in an isotropic melt, whereas the longitudinal axes of the molecules are mutually parallel. This is different from the state in the isotropic melt, where the orientation of the molecules is statistically distributed. The result is mechanical, electrical, and also optical properties which are anisotropic. In the "cholesteric" phases, there is an additional ordering principle comprised of a continuous helical variation of the orientation direction of the longitudinal axes of the respective molecules, which leads to special optical properties such as strong optical activity or selective reflection of light. In the so-called "smectic" phases, in addition to the above-described orientation ordering characteristic of the nematic state there occurs a regular disposition of the centers of gravity of the molecules in space, e.g., only along one spatial axis, or in other smectic modifications along two or even three mutually independent axes. These phases are nonetheless still fluid.
Disc-shaped molecules can form so-called "discotic" phases, in which either only the disc normals are mutually parallel, or the discs are disposed in regular or irregular fashion within columns. In this case, one speaks of "columnar structures".
A characteristic parameter of liquid crystal structures, and one which is very important for the present invention, is the "orientation ordering parameter", which is a measure of the level of orientation order. Its value lies between zero (completely random orientation as in the isotropic melt) and unity (perfect parallel orientation of all molecular longitudinal axes).
The widespread use of liquid crystal substances in industrial and technical products, e.g., display elements in calculators, wristwatches, and digital measuring devices, is based on their ability to easily change the orientation direction (which may be represented by the so-called "director") by applying an external electric, magnetic, or mechanical field. The changes in optical properties which are produced by such external fields can be utilized for information display, when such optical effects are combined with other components such as polarizers, cell walls, etc., in display devices. The cell walls serve to protect the fluid mesophases, and impose the required macroscopic shape on the liquid crystal film.
In recent years it has been recognized that for many areas of application it can be advantageous to combine the properties of liquid crystal phases with those of polymers. Polymer properties considered advantageous are good mechanical characteristics (which enable such substances to be processed into thin films with a stabile shape) and the occurrence of a solidification process (glass transition) which enables a predetermined orientation structure to be fixed. Another parameter, the glass temperature (T.sub.g), characterizes the existence of the solidified liquid crystal phase. Above this temperature, the polymer has a viscoelastic or cohesive elastic state.
Theories of the formation of liquid crystal phases in general and of the formation of such phases in polymer systems in particular, as well as experimental results, indicate that the means of producing liquid crystal polymers generally involve the use of rigid mesogenic structural units of the type characteristic for low molecular weight liquid crystals, along with flexible spacer groups and flexible chain molecules. In this way, a wide variety of structural schemes is possible. In the side-chain class of liquid crystal polymers, the mesogenic groups are attached to a flexible or semi-flexible main chain, with or possibly without a flexible spacer. The mesogenic groups used may comprise cylindrical shaped or disc shaped groups. The main chain itself may also contain mesogenic groups which are separated by flexible units. Copolymers characterized by various different spacers and/or mesogenic groups occurring within a single polymer may also form liquid crystal phases.
In addition to these side-chain liquid crystals, main-chain polymers can form liquid crystal phases under certain conditions. These conditions are, namely, that the chains are either completely comprised of rigid groups or of rigid and flexible groups. Copolymers comprised of different mesogenic groups and/or spacer groups also enable liquid crystal phases to be formed. The mesogenic groups may be somewhat cylindrical or rod-shaped. It is possible to adjust to some extent, in advance, the nature of the mesophases and the existence domain of these phases and of the glass state, by means of the structure of the mesogenic groups, the length and flexibility of the spacers, the flexibility of the main chain, and also the ordering of the sequence of units and length of the main chain.
Thus far virtually the only liquid crystal polymers which have been marketed are main chain copolymers with exclusively rigid units, or with predominantly rigid units. They have extremely high strength and stiffness parameters. The term used for these polymers is "self-reinforcing thermoplastics". They are used for industrial parts requiring outstanding mechanical properties. See Kirk-Othmer, "Encyclopedia of Chemical Technology", 3rd Ed., Vol. 14, pp. 414-21 (1984); Wendorff, J. H., Kunststoffe, 73:524-8 (1983); and Dobb, M. G., and McIntyre, J. E., Adv. Polylm.Sci., 60/61:61-98 (1984).
Polymers with both flexible and rigid units have not yet been employed in systems which have reached the market. The advantage lies in the high value of the orientation ordering parameter, compared to that of side chain liquid crystals. See Noel, C., Laupretre, F., Friedrich, C., Fagolle, B., and Bosio, L., Polymer, 25:808-14 (1984); Wunderlich, B., and Grebowicz, I., Adv. Polym. Sci., 60/61:1-60 (1984); and Kirk-Othmer, loc. cit.. Polymers with mesogenic side groups have attracted strong interest recently. See Clough, S.B., Blumstein, A., and Hsu, E. C., Macromolecules, 9:123 (1976); Tsekov, V. N. et al., Europ. Polym. J., 9:481 (1973); Strzelecky, L., and Libert., L., Bull, Soc. Chim. France, 297 (1973); Finkelmann, H., in "Polymer Liquid Crystals" (1982); Frenzel, J., and Rehage, G., Macromol. Chem., 184:1689-1703 (1983); Macromol. Chem. Rapid Commun., 1:129 (1980); Hoppner, D., and Wendorff, J. H., Angew. Makromol. Chem., 125:37-51 (1984); German OS 27 22 589, OS 28 31 909, OS 30 20 645, OS 30 27 757, and OS 32 11 400; and European OS 90 282.
U.S. Pat. No. 4,293,435 discloses an industrial use of the specific behavior of liquid crystal polymers which is associated with the transition to the vitreous state. In that application, information is stored by making use of conditions which change the configuration and orientation of the liquid crystal polymers in a predetermined manner, the conditions being, e.g., electric and magnetic fields, and/or pressure. This is also discussed in British Pat. No. 2,146,787. It is noted that the storage of the device in the solid state below the glass temperature (T.sub.g) implies that T.sub.g is above normal room temperature (T.sub.a), i.e., that the polymer system must be employed at temperatures on the order of 100.degree. C. above T.sub.a if it is desired to record the incoming information within a reasonable response time. Such temperatures are technically inconvenient. Over a period of time, they in fact result in decomposition of the polymer. According to British Pat. No. 2,146,787 these problems can be avoided if certain polymeric side chain liquid crystals are employed. It is then no longer necessary to maintain the temperature in a range below T.sub.g in order to preserve the device, but rather it should be possible to store the device for many years at temperatures above T.sub.g and below a temperature (T.sub.f) at which the polymer material begins to become fluid.
T.sub.f can be determined by following the transmission of light through a liquid crystal polymer between two crossed polarizing filters as the temperature is increased from T.sub.g. At a few degrees Centigrade below the smectic-to-isotropic phase transition the transparency will increase suddenly. This increase is due to the passage from an anisotropic (but less transparent) state to a highly birefringent (but transparent) state. The temperature above this temperature T.sub.f are referred to as the "fluid region". As the temperature is increased the transparency also increases, until it reaches a maximum of T.sub.m. T.sub.m marks the point at which the isotropic (clear) phase first appears.
Since the appearance of the isotropic phase leads to extinction of the light in the presence of the crossed polarizers, further increases in temperature are accompanied by a drop in transparency, as the isotropic regions within the plastic mass increase in size. Eventually the so-called "clear temperature" is reached, at which the last remnants of the structure responsible for the birefringence have disappeared.
In British Pat. No. 2,146,787, a device is claimed which is comprised of a layer of material which contains a liquid crystal polymer with a mesogenic side chain; is further comprised of devices to thermally convert at least part of the material from the viscous state wherein the temperature of the material is in the range T.sub.g to T.sub.f to the fluid region; and is further comprised of devices to influence at least part of the material in the fluid region in order to produce a selective change in the texture of the molecules, thereby imparting information which is retained even after the material is cooled out of the fluid region and returns to the viscous state. Accordingly, an essential precondition with the British Patent is that the temperature parameters of the polymer material obey the relation T.sub.f &gt;T.sub.a &gt;T.sub.g. A device is also described in which the material layer contains a liquid crystal polymer with a smectogenic side chain. Particularly preferred are polymeric liquid crystals of the polysiloxane type, with diphenylcyanogen side chains or benzoic acid ester side chains.
As in the past, there is currently great interest in optical storage media which have high display density and are capable of reversible storage. The above-described solutions to the problem of optical data storage represent relatively narrow technical answers. Thus, the device according to British Pat. No. 2,146,787 depends on the use of liquid crystal side chain polymers with the crucial precondition that the temperature be chosen such that the polymer material is in a viscous region. The refinements disclosed therein extend to polysiloxane liquid crystals, preferably with diphenylcyanogen side chains or benzoic acid ester side chains. The information storage is not reliable, due to the molecular mobility which is present, the finite relaxation times associated therewith, and the possibility of influencing the system, e.g., by spurious fields. Also, it would be desirable to have a system which could be implemented without such tight restrictions on parameters.